Ceramic materials are widely used for ballistic protection with the purpose of overcoming the high power penetration of projectiles, such as armor piercing (AP) projectiles, due to their relatively low density, high rigidity and high compression strength. See C. Barry Carter, M. Grant Norton, Ceramic Materials Science and Engineering, Spinger Science, 2007. Usually, ceramics are used in structures for ballistic protection in the form of plates, tiles, pellets or specially shape ceramic elements.
The significant disadvantage of these ceramic components is their low fracture toughness and the formation of a wide net of relatively long cracks during fracture. Such behavior of ceramic components reduces their ability to protect against not only around the local impact zone, but also around the wide area, which is affected by propagated cracks.
The impact on the ceramic component by a projectile generates the propagation of compression stress waves, which are reflected back from the free surfaces of the protection plate as tensile waves.
The ceramic materials have low tensile strength and usually these tensile stresses lead to the fracture of the ceramic component. It is therefore reasonable to assume that pre-stressing (initial compression) the ceramic component in a ballistic protection structure, should decrease the level of reflected tensile stresses after impact, and improve the protective ability of the ceramics component in a protection structure.
D. Sherman, J. PHYS IV FRANCE 7 1997, showed that inducing the compression stresses in the ceramics by confined frame reduces significantly their damage from impact.
T. J. Holmquist, International Journal of Impact Engineering, November 2003, evaluated the response of ceramic targets with and without pre-stress subjected to projectile impact. The results demonstrate that ballistic performance of confined ceramic targets can be improved by pre-stressing the ceramic.
U.S. Pat. No. 6,389,594 describes that an applying of the high isostatic pressure on a monolith ceramics wrapped by a composite materials impregnated with adhesives to cure it, induces the internal stresses into the ceramics and improves the multi hit capabilities of the armor.
U.S. Pat. No. 6,601,497 discloses an armor component that includes a tile having a perimeter, with optional recess to encase a wrapping material wrapped around the perimeter of the tile. The purpose of wrapping the tile is to prevent lateral expansion of the tile if it is hit. The wrapping material pre-stresses the tile, however the technique used here limits the degree of tensioning of the wrapping material by relatively low ability of tensioners and delivery system for the wrapping materials, and does not take an advantage of the tensile ability of them. The level of pre-stressing of the tile depends on the perimeter geometry of the tile, where the circular perimeter provides a uniform pressure on the tile recess. The straight sections of the tile perimeter will not supply any pressure on the tile, so this technology shouldn't be effective for monolithic ceramic plates that usually have straight perimeter lines, like personal protection ceramic plates.
U.S. Pat. No. 7,300,893 describes an armor including strain rate hardening elastomers. An option to wrap the ballistic cloth or fibers around the ceramic tiles is suggested in this invention as a way to encapsulate the elastomer, and not for pre-stressing of the ceramic plate.
U.S. Pat. No. 7,827,899 describes an armor panel to protect a body from an incoming projectile, comprising the ceramic or glass-ceramic plate and backing polyethylene cloth, encapsulated within by a wrapping layer and bonded together by applying an external pressure on this wrapping layer. An armor panel, produced by this way, contains the externally wrapped layer, but this layer doesn't induce any significant pre-stressing of the internal ceramic plate.
The above prior art relates to pre-stressing anti-ballistic flat ceramic articles.
It is an object of the present invention is to provide anti-ballistic pre-stressed curved monolithic plates/tiles and a method of producing same.
It is a further object of this invention to provide a method of preparing pre-stressed anti-ballistic curved plates/tiles.
Anti-ballistic articles containing pre-stressed curved plates/tiles according to the present invention have superior ballistic characteristics including significant reduction of damages to the plate when hit, thus, providing armor plate with an enhanced multi-hit capabilities. In addition, anti-ballistic articles containing pre-stressed curved plates of the present invention are of relatively lower weight and decreased thickness than conventional ballistic articles for achieving the same affect.
Further objectives, features and advantages of the invention will become apparent from the following detailed description taken in conjunction with the following figures.